1. Field of the Invention
The present invention relates to a method and apparatus for partially encrypting speech packets, and more particularly, to a method and apparatus for selecting partial encryption sets used for a partial encryption technique in which compressed speech packets are partially encrypted to obtain the same effect as a full encryption technique.
2. Discussion of Related Art
With the development of wireless communication technology and users' demand for mobility, low-power mobile terminals have been required more and more. Although wireless communication environments need higher security levels than wired communication environments, when encryption services that require large amounts of operations are applied to the wireless communication environments, the life spans of batteries of mobile terminals are shortened. This becomes more problematic in ultra low-power communication environments, such as wireless sensors and ad-hoc networks.
One of methods that have been proposed to overcome the problem of the encryption services is a partial encryption technique. According to the partial encryption technique, only information that is sensitive to human perceptual characteristics is extracted and encrypted instead of encrypting all compressed multimedia streams. The partial encryption technique has the same effect in information security as a full information encryption technique. Since the partial encryption technique can lessen the amount of information to be encrypted, the amount of encryption operation can greatly decrease. In the partial encryption technique, a partial encryption set refers to a combination of accumulated bits to be partially encrypted among compressed speech packets. Thus, selecting a partial encryption set having the smallest size within a range of ensuring security is the most essential to the partial encryption technique.
FIGS. 1A and 1B are block diagrams showing a full encryption technique and a partial encryption technique, respectively.
Referring to FIG. 1A, which illustrates a full encryption technique, a compression unit 100 compresses a speech signal into speech packets, and an encryption unit 101 encrypts the entire compressed speech packets to produce encrypted speech packets. In contrast, referring to FIG. 1B, which illustrates a partial encryption technique, a compression unit 110 compresses a speech signal into speech packets, and a packet segmentation unit 111 segments the compressed speech packets into perceptually sensitive information and insensitive information. Thus, an encryption unit 112 encrypts only the perceptually sensitive information. A packet assembly unit 113 reassembles the encrypted information and the unencrypted information to produce a partially encrypted speech packet. In the partial encryption technique, the packet segmentation unit 111 and the packet assembly unit 113 extract and reassemble the perceptually sensitive information based on a partial encryption set. The security and efficiency of the partial encryption technique depend on the partial encryption set.
A conventional technique related with the above-described secure and efficient partial encryption set is described by A. Servetti and J. C. De Martin, in “Perception-based partial encryption of compressed speech”, IEEE Trans. Speech and Audio Processing, vol. 10, no. 8, pp. 637-643, November 2002. In this conventional technique, two partial encryption sets are selected for an ITU-T G.729 codec and the security of each of the two partial encryption sets is evaluated. On analysis of the evaluation results, a high-protection set that corresponds to 45% of the total bit stream exhibits the same security level as in a full encryption technique, and a low-protection set that corresponds to 30% of the total bit stream is capable of nearly removing the understanding of an encrypted speech signal.
Table 1 shows bits allocated to the ITU-T G.729 and the conventional partial encryption sets.
TABLE 1Low-ParameterSymbolsG.729High-protection setprotection setLSPL0100L1775L2553L3500PitchP0100P1875P2533GainGA1332GB1442GA2332GB2442ResidueS1400C11300S2400C21300Total number of bits80 bits36 bits24 bitsEncryption rate45%30%
One frame of the G.729 codec consists of 80 bits, and the G.729 codec includes a line spectral pair (LSP) parameter, a pitch parameter, a gain parameter, and a residual parameter. As shown in Table 1, the conventional high-protection set consists of 45% of the 80 bits (or 36 bits), and the conventional low-protection set consists of 30% of the 80 bits (or 24 bits).
Conventionally, the high-protection set and the low-protection set are selected as follows. Initially, a set selected for an unequal error protection (UEP) method (hereinafter, a “UEP set”) is cited and used as the high-protection set. The cited UEP set is selected by measuring objective and subjective speech qualities of each bit of the bit stream of the G.729 codec. Also, the low-protection set is selected via an informal listening test that is additionally programmed.
Although the above-described conventional technique provides the method of partially encrypting compressed speech packets and partial encryption sets, the partial encryption sets have the two following efficiency problems.
1. A Problem about a Difference in a Minimization Target Between the Partial Encryption Set and the UEP Set
In order that the conventional high-protection set may be applied to a UEP method, the most sensitive bits are combined by measuring the objective and subjective speech qualities of a single bit to generate the high-protection set. The EUP method is a technique of further adding an error correction function to information sensitive to perceptual characteristics in a multimedia stream and less adding the error correction function to information insensitive to the perceptual characteristic so as to elevate the entire communication qualities in various loss environments. However, since the UEP set is different in uses from the partial encryption set, standards for selecting the UEP set and the partial encryption set should be different. The UEP method is directed to selecting bits sensitive to errors and protecting mainly the bits to minimize the degradation of the entire communication quality. In contrast, the partial encryption method is directed to maximizing distortion of a speech signal to minimize residual intelligence of a speech signal using the smallest partial encryption sets. Accordingly, the UEP method and the partial encryption method are aimed at minimizing the opposite targets. Due to the difference in the minimization target, it is necessary to improve the conventional partial encryption set as follows.
1) The UEP method must include a large portion of a packet in order to satisfy a speech quality threshold value, and a UEP set that consists of many bits includes too many residual components to be directly used as a partial encryption set. Thus, only a far smaller number of bits may be used to remove residual intelligence.
2) The UEP set is selected by measuring single-bit sensitivity on the assumption of low-loss environments. However, environments in which respective bits suffer from losses at the same time need to be considered in a high-loss environment, such as partial encryption.
2. A Problem about Criticality of a Parameter MSB
The conventional low-protection set is selected via an informal listening test that is additionally programmed to select a partial encryption set. However, a graphical user interface (GUI) of the program has a scroll bar for selecting the number of bits to belong to a partial encryption set for each parameter. The number of bits selected by the scroll bar corresponds to the number of bits of the corresponding parameter in a direction from a most significant bit (MSB) to a least significant bit (LSB). The number of bits is selected for each parameter on the assumption that the MSB of the parameter exhibits more loss distortion than the LSB of the parameter. However, each parameter of a speech codec is obtained using a vector quantization method and a quantization method peculiar to the speech codec instead of a simple scalar quantization method. For example, a G.729 codec uses various quantization methods shown in Table 2. Accordingly, the conventional assumption that an MSB is always more sensitive to loss than an LSB is erroneous, thus resulting in a reduction in the efficiency of the selected partial encryption set.
TABLE 2ParameterSymbolQuantizationLSPL0, L1, L2, L3Vector quantizationPitchP0Quantization peculiar to G.729P1Scalar quantizationP2Differential scalar quantizationGainGA1, GB1, GA2, GB2Vector quantizationResidueS1, C1, S2, C2Quantization peculiar to G.729